1. Field of the Invention
The present invention relates to a blade for an axial fluid machine, and more specifically, to blades for an axial fluid machine for lowering a secondary flow loss which is caused when a blade cascade disposed along the axial direction of a rotational shaft passes through a working fluid to thereby improve the efficiency the blade cascade.
2. Discussion of the Background
In general, although there are various types of prime movers such as an axial type, a radial flow type, a centrifugal type, a volume type and the like in a fluid machine, since a high output can be obtained from the axial type prime mover among them, it has been used as an ultra-large prime mover for an air compressor, a gas turbine, a steam turbine and the like which are used with air craft and power generation plants.
As shown in FIG. 14, an axial fluid machine, for example, an air compressor is arranged such that it is accommodated in a casing 1, and compressor stages 4 each composed of the combination of a stationary blade 2 and a moving blade 3 are disposed along the axial direction of a rotational shaft 5. Atmospheric air 7 sucked from an inlet 6 is compressed by the moving blade 3, high pressure air passing a first compressor stage is then guided to the next compressor stage 4 through the moving blade 3, and the resulting high pressure air 8, which has been compressed to a predetermined pressure, is supplied to a gas turbine, not shown, from an outlet 9.
Further, as shown in FIG. 15, the axial fluid machine, for example, a steam turbine, is arranged such that it is accommodated in a casing 10 and has turbine stages 17 each composed of the combination of a stationary blade 13, which is clamped between a diaphragm outer ring 11 and a diaphragm inner ring 12, and a moving blade 14 planted on the disk 16 of a rotational shaft 15. The turbine stages 17 are disposed along the axial direction of a rotational shaft 15, stream 18 is expanded by the stationary blades 14 and the expanding force thereof is applied to the moving blades 14 so as to rotate the same to thereby utilize the rotational force thereof. Further, steam leaking at a time when the stream 18 passes through the turbine stages 17 is sealed by the labyrinths 19 planted on the diaphragm inner ring 12.
Incidentally, when the high pressure air 8 passes through the stationary blades 2 and the moving blade 3 in the axial air compressor or when the stream 18 passes through the moving blade and the stationary blades 13 in the axial steam turbine, various losses are caused, which result in a reason for lowering a blade cascade (train) efficiency as a blade cascade loss.
The blade cascade loss includes a profile loss caused by the shape of a profile itself, a clearance leakage loss caused by the clearance between a blade tip and a flow passage wall, and an endwall loss caused by the existence of the inner peripheral surface and the outer peripheral wall of a flow passage. Among them, the endwall loss is a large factor in the reduction of the blade cascade efficiency.
The wall surface loss is mainly caused by a swirl resulting from a secondary flow generated in the blade cascade and the boundary layer exfoliation of the flow passage wall caused by the swirl. Typical examples of the swirl resulting from the secondary flow and the boundary layer exfoliation are common to the axial air compressor and the axial stem turbine, and it is known that the secondary flow is generated by the behavior of a main stream passing between the blade cascade (although air is a working fluid in the air compressor and stream is the working fluid in the stream turbine, each of these working fluids is referred to as the main stream in the following description).
The main stream flows along a blade profile on the intermediate portion side of a blade in the lengthwise direction thereof when it passes between the blade cascade, whereas the secondary stream is a stream which flows in a direction which intersects the main stream flowing on the side of an intermediate blade height portion on a blade tip side and a blade root side and is generated by a pressure difference between a blade and an adjacent blade.
When the main stream intersects the secondary stream, it is accompanied by a swirl which is generated as shown in FIG. 16 and the swirl will be generated in a short time. That is, when main streams 20a, 20b accompanied by inlet boundary layers 20a.sub.1, 20b.sub.1 flow into the flow passages 22a, 22b between blades 21a, 21b, they collide against leading edges 23a, 23b and then generate swirls 24a, 24b.
The swirls 24a, 24b are divided into concave side horseshoe-shaped swirls 24a.sub.1, 24b.sub.1 and convex side horseshoe-shaped swirls 24a.sub.2, 24b.sub.2. When the convex side horseshoe-shaped swirls 24a.sub.2, 24b.sub.2 flow along the convex sides 25a, 25b of the blades 21a, 21b which are made to a negative pressure, they flow to trailing edges 26a, 26b while gradually growing by rolling the boundary layers of the flow passages 22a, 22b.
On the other hand, when the concave side horseshoe-shaped swirls 24a.sub.1, 24b.sub.1 flow toward the convex sides 25b, 25c of the adjacent blade 21b, 21c together with a secondary stream due to the pressure difference between the concave sides 27a, 27b of the blades 21a, 21b which are under a positive pressure and the convex sides 25b, 25c of the adjacent blades 21b, 21c which under a negative pressure, they are greatly enlarged by rolling the boundary layers of the flow passages 22a, 22b, and then the swirls 24a.sub.1 and 24a.sub.2 merge with the convex side horseshoe-shaped swirls 24a.sub.2, 24b.sub.2 as flow passage swirls 24a.sub.3, 24b.sub.3.
As described above, it is to be understood that, as a secondary stream swirl as a whole, the swirls 24a, 24b, which are generated by the collision of the main streams 20a, 20b against the leading edges 23a, 23b of the blades 21a, 21b, are divided into the concave side horseshoe-shaped swirls 24a.sub.1, 24b.sub.1 and the convex side horseshoe-shaped swirls 24a.sub.2, 24b.sub.2, that the concave side horseshoe-shaped swirls 24a.sub.1, 24b.sub.1 are greatly enlarged and are added to the flow passage swirls 24a.sub.3, 24b.sub.3 and that the convex side horseshoe-shaped swirls 24a.sub.2, 24b.sub.2 are greatly enlarged while they flow along the convex sides 25a, 25b.
The secondary stream swirl exhibits the flow lines of the main streams 20a, 20b which pass in the vicinity of the wall surfaces of the flow passages 22a, 22b and is a main reason why the blade cascade efficiency of the blades 21a, 21b is lowered. Accordingly, it is necessary to suppress the secondary stream swirl.
For example, Japanese Patent Publication No. 56-19446 discloses a method of suppressing the secondary stream swirl. As shown in FIG. 17, the technology of this publication shows theoretical arrangement in which projecting blade portions 30 are formed at the leading edge 29 of an effective blade portion 28 over distances 1a from flow passage walls 31 and the lateral cross-sectional shape of each of the projecting blade portions 30 is formed such that a projecting blade portion convex side 32a is caused to coincide with an effective blade portion concave side 32b and a projecting blade portion concave side 33a is bulged outward more than the effective blade portion convex side 33b as shown in FIG. 18. Further, the intersection point S10 of the projecting blade portion 30 with the flow passage wall 31, the intersection point S20 of the projecting blade portion 30 with the effective blade portion 28 and the projecting point S30 of an intersection point S20 to the flow passage wall 31 shown in FIG. 17 are caused to correspond to the points S10, S20 and S30 shown in FIG. 16, respectively.
In such technology, the convex side horseshoe-shaped swirl of the secondary stream swirl is suppressed to a low level by making the pressure of the effective blade portion convex side 33b higher than that of the projecting blade portion concave side 33a along a blade chord c as shown in FIG. 19 in such a manner that the projecting blade portions 30, 30 are formed toward the leading edge 29 of the effective blade portion 28 toward the flow passage walls 31, 31 sides and the code length of the projecting blade portions 30, 30 is increased with respect to the chord length of the effective blade portion. That is, a pressing force is applied to the projecting blade portion convex side 33a from the effective blade portion convex side 33b to thereby suppress the growth of the convex side horseshoe-shaped swirl.
As described above, the prior art shown in FIG. 17 has an excellent advantage that the growth of the convex side horseshoe-shaped swirl is suppressed to the low level. However, since the pressure difference between the effective blade portion concave side 32b and projecting blade portion concave side 32a, and the projecting blade portion convex side 33b is made more higher than a conventional pressure difference shown in FIG. 19, the growth of a flow passage swirl from the concave side of a blade to the convex side of the other adjacent blade is more greatly promoted. As a result, the prior art shown in FIG. 17 provides a disadvantage that a flow passage swirl which will be greatly grown from the concave side horseshoe-shaped swirl in a short time cannot be suppressed and the blade cascade efficiency cannot be increased.